Passive contactless integrated circuits are usually used in RFID (Radio Frequency Identification) applications and can be of the inductive coupling type or of the “electrical coupling” type.
The passive integrated circuits of the first (inductive coupling) type comprise an antenna coil, send data by load modulation and are powered by inductive coupling in presence of a magnetic field whose frequency is usually about 10 MHz. These integrated circuits are for example described by the standards ISO/IEC 14443A/B, ISO/IEC 13693 which provide a working frequency of 13.56 MHz.
The passive integrated circuits of the second (electrical coupling) type are electrically powered by a UHF electric field oscillating at several hundreds of MHz, and send data by modulating the reflection rate of their antenna circuit (this technique is called “backscattering”). These integrated circuits are for example described by the industrial specification EPCTM-GEN2 (“Radio-Frequency Identity Protocols Class-1 Generation-2—UHF RFID Protocol for Communications at 860 MHz-960 MHz”) in the course of standardization. They are generally used in long range applications, wherein the distance between the integrated circuit and a station for sending/receiving data emitting the electric field, commonly called a “reader”, can reach several meters.
FIG. 1 schematically shows the architecture of an integrated circuit IC1 of the second type. The integrated circuit comprises a dipole antenna circuit AC, a communication interface circuit CICT, a control unit CTU1, a non-volatile memory MEM of the EEPROM type (electrically erasable and programmable memory), a charge accumulation booster circuit HVCT and an oscillator OSC supplying a clock signal CK to the unit CTU1.
The circuit CICT ensures the reception and transmission of data via the antenna circuit. It receives from the control unit CTU1 data DTx to be sent via the antenna circuit or supplies to the unit CTU1 data DTr received via the antenna circuit. It also supplies a voltage Vcc ensuring the power supply of the integrated circuit and generated from antenna signals S1, S2 appearing in the antenna circuit AC in presence of an electric field E emitted by a reader (not shown). The voltage Vcc is typically of about one Volt to a few Volts.
The booster circuit HVCT supplies, from the voltage Vcc, a high voltage Vhv necessary for operations of writing data in the memory, typically about 10 to 15 V. The voltage Vhv is usually applied to the memory through a switch circuit SCT which is controlled by the unit CTU1.
When a write command is received, the unit CTU1 activates the booster circuit HVCT, applies to the memory MEM a write address ADW and data to be written DTW, and activates the switch circuit SCT so that a voltage pulse Vpp is applied to the memory. These operations can be repeated several times according to the number of write cycles to be performed, while leaving the booster circuit activated.
A drawback of this integrated circuit, inherent to its passive nature, is that the supply voltage Vcc can have critical attenuations caused by variations of the intensity of the electric field E from which it is extracted. There is a similar drawback on passive integrated circuits of the inductive coupling type, when the intensity of the magnetic field and/or the rate of inductive coupling with the reader are insufficient. These attenuations occur for example when the user of the integrated circuit, for example the holder of a contactless chip card comprising the integrated circuit, moves the card/the integrated circuit away from the reader during a communication between the reader and the integrated circuit. These attenuations of the supply voltage are problematic when they occur during the writing of data in the memory, because they can affect the high voltage Vhv. Thus, if the voltage Vhv has an insufficient level when the integrated circuit triggers the writing of the memory, memory cells might not store the data expected or might be set in an intermediary state which is neither the programmed state nor the erased state, corresponding to a corruption of data stored in the memory cells.
The U.S. Pat. No. 6,288,629 suggests to monitor the supply voltage Vcc when data are written in the memory, and to raise an indicator flag when the voltage Vcc varies below a critical threshold during the write phase. When the write phase ends, the integrated circuit reads the flag and, if the flag indicates that the voltage Vcc has become lower than the critical threshold during the writing phase, the integrated circuit communicates this information to the reader which has emitted the write command.
According to studies carried out by the present inventors, including various simulations aiming at observing the behavior of a contactless integrated circuit in operating conditions corresponding to concrete applications, this method can lead to diagnose that a write operation is faulty whereas it has developed properly. Indeed it turns out that, in various operating conditions, the supply voltage Vcc can have attenuations, or even short micro disconnections, which do not affect the writing process.